(1) Field of the Invention
The present invention relates to a tilt error corrective scanning optical system mainly used in a laser printer or the like for eliminating scanning pitch slippage of scanning lines in an auxiliary scanning direction.
More particularly, the present invention relates to a tilt error corrective scanning optical system comprising a linear image forming optical device for causing a beam emitted from a light source to form a linear image on a deflecting and reflecting surface of deflecting means, and a scanning image forming optical device for causing the beam reflected and deflected by the deflecting means to form an image on a scanned object.
(2) Description of the Prior Art
In recent years, there is an increasing demand for the laser beam printer in accordance with diversification and development of office automating machines because of becoming smaller and cheaper as well as advantage of high speed printing. With the laser beam printer including deflecting means such as a polygonal mirror for deflecting a beam emitted from a light source, for example, there occur tilt errors with respect to a direction perpendicular to a scanning plane (in the case of a polygonal mirror, tilt errors with respect to an axis of revolution of reflecting surfaces) which are caused by manufacturing errors, assembly errors or vibrations at times of rotation of the mirror.
The beam reflected by a deflecting and reflecting surface having such errors results in an image forming position on a scanned object slipped or deviated in an auxiliary scanning direction and a nonuniformity in the scanning line pitch. In the case of a recording apparatus such as the laser beam printer, for example, the nonuniformity in the scanning line pitch deteriorates the quality of recorded images.
The aforesaid tilt error corrective scanning optical system is designed for eliminating the nonuniformity in the scanning line pitch. The beam emitted from the light source is once converged by the linear image forming optical device in a direction perpendicular to the scanning plane to form a linear image on a deflecting and reflecting surface of the deflecting means. The beam from a point of deflection and reflection is restored in the above direction by the scanning image forming optical system to form a conjugate image on the scanned object. This process substantially precludes the influence of the tilt errors of the deflecting and reflecting surface.
On the other hand, the tilt error corrective optical system causes the beam to form on the scanned object an image having a height proportional to an incident angle of the beam on the scanning image forming optical device, so that the beam scans the object at a constant scanning speed.
In this specification the term "scanning plane" means a plane formed by a time series set of the scanning beam, namely a plane including main scanning lines on the scanned object and an optical axis of the tilt corrective scanning optical system.
Various constructions of the tilt corrective scanning optical system have been proposed heretofore.
One example is disclosed in U.S. Pat. No. 3,750,189. This construction comprises a scanning image forming optical device including a beam shaping optical element such as a cylindrical lens for restoring the beam into a circular form, which beam has been converged into a line by a linear image forming optical element and reflected by deflecting means, and a converging optical element for converging the restored beam to form an image on the scanned object.
Where the beam is restored by the beam shaping optical element as in the above construction, the restrictive condition to regain the circular beam is imposed on the beam shaping optical element. As a result, there is a reduced freedom for improving distortion characteristics provided for the converging optical element to realize the constant scanning speed of the beam and image forming characteristics for forming images on the scanned object. In order to realize excellent distortion and image forming characteristics, the scanning image forming optical device requires many lenses, thereby complicating the optical device construction.
The Japanese patent application laid open under No. 50-93720 discloses a proposal for improving the foregoing prior construction. In this construction, a beam shaping optical element such as a cylindrical lens as noted above is interposed between the converging optical element and the scanned object.
With this construction, the beam shaping optical element must be located close to the scanned object in order to produce high quality images. It is therefore necessary for the beam shaping optical element to be long in the main scanning direction, which is contrary to a compact construction.
A further known construction is disclosed in U.S. Pat. No. 4,379,612 wherein the scanning image forming optical device disposed between the deflecting means and the scanned object comprises a single spherical lens and a single lens including a toric surface, the latter being disposed at a side of the single spherical lens opposed to the scanned object. This scanning image forming optical device has the distortion characteristics for realizing the constant speed scanning of the beam as well as the function to correct the tilt errors of the deflecting and reflecting surfaces in cooperation with the linear image forming optical system.
Although this construction realizes a compact optical system, only a limited freedom is provided thereby. For example, it is difficult to enlarge the field angle for increasing the scanning range of the beam where both the distortion characteristics for realizing the constant speed scanning of the beam and the function to correct the tilt errors of the deflecting and reflecting surfaces are maintained in an excellent state.